The Non-Immigrant O visa stands for Other and is used for situations such as volunteering, spousal or child visitation, sporting event attendance, medical treatment, legal witness, or attendence for state enterprises or social welfare organizations. It used to be used for retirement as well, but a new category the *Non-O-A Longstay" is used for retirement.
For those who are married, the Non-O Spousal Visitation visa is a convenient one, which can be issued in series, is good for 1 year of 90-day multiple-entries (effectively ~15 months in length), and costs 5,000 THB. Depending on the Thai consulate, no financial documents need be required. Currently the Savannakhet, Laos and Ho Chi Minh City, Vietnam Thai Consulates issue these visas, though it shouldn't be difficult to get this at the Hanoi, Vietnam or Yangon, Myanmar consulates as well (ask in advance, of course). Other Laos locations and Cambodia and Malaysia have reputations for being more difficult. Farther afield, possibly the Singapore and Indonesian Thai consulates would be accommodating.
Discussion Groups for Thai Visas
For the latest trip reports, the Thai Visa and Teak Door forums are a place for information (though double check any older discussions with the latest updates).
Calendars and Holidays for the Thai Consulates
Make sure to check the calendar of the specific consulate before booking flights and accommodation. We've been caught out twice in this regard: once in Vancouver, Canada for Chulalongkorn Day, and once in Vientiane, Laos for Women's Day (a holiday there). Consult the calendars of both Thailand and whatever country the Thai Consulate is in. And regarding Thailand, they can add public holidays on a whim, with very little notice, so be aware of that (and good luck if plane tickets are purchased in advance).
Note that for our experience with Women's Day, it wasn't that we arrived on Women's Day, but that so many other foreigners did that the day after Women's Day the gates to the consulate closed 30 minutes early and we were caught out by it.
Booking Flights and Accommodation
Discount flights are more sensitive so booking up to six months in advance is suggested. Google has a very nice flight fare tracking tools that can give some sense of how cheap or expensive a fare is relative to days, weeks, and months around a given set of dates.
Accommodation can be done three months in advance or so (more if seeking out rare or exceptionally popular accommodation). Booking.com or Agoda are reasonable approaches. Our experiences with Airbnb are so mixed we simply don't use them anymore.
Visas, Visa Exemptions, and Visa-on-Arrivals
When travelling to the foreign country where the Thai Consulate is located, visas or visa exemptions are a part of the planning. In Vietnam this must be done in advance as visa exceptions and visa-on-arrivals are not widely available (for many nationalities). Currently there is a 30-day evisa available for Vietnam for many nationalities, and is the cheapest option at $25 USD (with no stamping fee). Laos has a 1,500 THB visa-on-arrival.
Ensure that you carry additional passport-sized (and appropriate) photos. These days eyeglasses are not allowed on passport photos. Also know that biometrics will be taken at many entries (e.g., Singapore, Thailand).
Local Sim Cards
Even for a few days, picking up a cheap sim card makes sense if telephone or internet service might be needed while in the country. It's usually quite cheap. Get one at the airport on disembark.
This can be done on arrival or better, on departure, depending where one is heading or departing from. Sometimes it is even easier to exclusively use an ATM for currency exchange, though not common.
While waiting for one's visa, it makes sense to take in the sights and cultural activities. Wikitravel has a number of suggestions (provided you are not going to Savannakhet which has zero sights and activities).
Hydration and Appropriate Clothing
For many climates (especially the tropics) it is wise to keep out of the sun, hydrate regularly, and take it easy, as tramping around can result in sunstroke / heatstroke, and/or dehydration.